linux-kernel - Re: [PATCHv3 3/3] dynamic_debug: Add a flag for dynamic event tracing

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Date:   Tue, 9 Nov 2021 10:49:41 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sai Prakash Ranjan <quic_saipraka@...cinc.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        <quic_psodagud@...cinc.com>, Marc Zyngier <maz@...nel.org>,
        <gregkh@...uxfoundation.org>, <arnd@...db.de>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        <mingo@...hat.com>, <jbaron@...mai.com>, <jim.cromie@...il.com>
Subject: Re: [PATCHv3 3/3] dynamic_debug: Add a flag for dynamic event
 tracing

On Tue, 9 Nov 2021 17:38:21 +0530
Sai Prakash Ranjan <quic_saipraka@...cinc.com> wrote:

> Debugging a specific driver or subsystem can be a lot easier if we can
> trace events specific to that driver or subsystem. This type of
> filtering can be achieved using existing dynamic debug library which
> provides a way to filter based on files, functions and modules.
> 
> Using this, provide an additional flag 'e' to filter event tracing to
> specified input.
> 
> For example, tracing all MMIO read/write can be overwhelming and of no
> use when debugging a specific driver or a subsystem. So switch to
> dynamic event tracing for register accesses.
> 
> Example: Tracing register accesses for all drivers in drivers/soc/qcom/*
> and the trace output is given below:
> 
>   # dyndbg="file drivers/soc/qcom/* +e" trace_event=rwmmio
>     or
>   # echo "file drivers/soc/qcom/* +e" > /sys/kernel/debug/dynamic_debug/control
>   # cat /sys/kernel/debug/tracing/trace

FYI, it's best to use /sys/kernel/tracing, as the debug/tracing is only
there for backward compatibility.

>     rwmmio_read: rpmh_rsc_probe+0x35c/0x410 readl addr=0xffff80001071000c
>     rwmmio_read: rpmh_rsc_probe+0x3d0/0x410 readl addr=0xffff800010710004
>     rwmmio_write: rpmh_rsc_probe+0x3b0/0x410 writel addr=0xffff800010710d00 val=0x3
>     rwmmio_write: write_tcs_cmd+0x6c/0x78 writel addr=0xffff800010710d30 val=0x10108

I'd much rather have a module name or something attached to the event that
ca be filtered on via the trace event filters, than having it determined by
some side effect done in another directory.

-- Steve